The storage hierarchy continues to evolve, from early use of punch cards to current high-performance flash devices. Though there are numerous media options, the classic storage hierarchy is often described as hard disk drives (HDDs) at the bottom providing slow, persistent storage and dynamic random access memory (DRAM) at the top providing fast, volatile storage. In the 1990s, flash-based solid state devices (SSDs) became available with latency and price between DRAM and HDD, though with limited endurance. As SSD capacity and price continue to improve, SSDs usage is transitioning from primarily a caching layer between DRAM and HDDs to displacing HDDs in certain situations. The new storage media under development present new options for improving the storage hierarchy.
Two main categories of new storage media have been the focus of the investigation due to their maturity and potential impact: non-volatile memory express (NVMe) and non-volatile dual in-line memory modules (NVDIMMs). While flash-based SSDs have become widely used, it has been noted that the current communication protocols impose a large overhead that hides the low latency of flash itself. The NVMe protocol not only reduces communication overheads but also adds deeper queues, more advanced interrupt mechanisms and lock-free parallelism. NVDIMMs offer the potential to have access times close to DRAM, byte level read/writes and persistent state across power outages. Because of the pricing of these products, they are likely to be considered for caches/tiers in the coming years instead of replacing legacy media immediately.
For both NVMe and NVDIMMs, there are numerous options under development in industry. Examples include 3D XPoint NVMe and NVDIMMs announced by Intel and Micron in 2015. Using new hardware, it is claimed that 3D XPoint will have a greatly extended lifespan, several orders of magnitude greater than traditional flash. Hereinafter NVMe with 3D XPoint may be referred to simply as NVMe, though NVMe is more accurately a protocol than a device. NVMe products are coming to market, while NVDIMM products are in an earlier stage of development.